This invention relates generally to improvements in gas turbine engines of the type which utilize compressor bleed air to provide air buffer sealing of oil sump seals. More specifically, this invention relates to a bleed air seal arrangement which provides improved buffer sealing at low engine power conditions.
Multiple spool gas turbine engines are generally known in the art, wherein at least two turbocompressor rotating groups are provided in association with a combustor. Each turbocompressor rotating group comprises a compressor stage and a turbine stage mounted on a common spool or shaft, with the shafts of separate rotating groups being arranged in concentric relation to each other. In a typical twin spool engine, a high pressure spool includes a compressor stage and a turbine stage disposed on opposite sides of the engine combustor, and rotatably interconnected by a hollow shaft which rotatably receives the shaft of a low pressure spool including a compressor stage and a turbine stage. In operation, the compressor stages of the low and high pressure spools provide dual stage compression of air which is supplied to the combustor for combustion with a suitable fuel. The hot gases of combustion are then expanded in series through the turbine stages of the high and low pressure spools, respectively, to provide an engine power output. One advantage of multiple spool gas turbine engines of this general type is that such engines can accelerate rapidly in order to accommodate increased power output requirements.
Gas turbine engines of the multiple spool type include a significant number of rotating and related bearing components which require lubrication for continued engine operation. In this regard, oil lubrication systems are well-known for delivering lubricant to selected bearings and related structures throughout the engine. Sump seals having a labyrinth or similar configuration are normally provided to prevent leakage of lubricating oil into the main flow path of air and combustion gases through the engine. Buffer seal arrangements have been proposed to pressurize engine sump seals in order to decrease the likelihood of oil leakage.
More specifically, in a typical buffer seal arrangement in a multiple spool gas turbine engine, a small portion of the compressed air produced by the compressor stage of the low pressure spool is diverted or bled into a buffer chamber within the engine. This bleed-off portion of the compressed air is commonly referred to as bleed air and is ducted from the buffer chamber to appropriate locations adjacent sump seals to provide a pneumatic pressure barrier intended to prevent undesired oil leakage. However, during some engine operating conditions, particularly such as relatively low power and/or transient operating conditions, the pressure of the bleed air can be insufficient to assure that no oil leakage past the sump seals will occur. Indeed, during rapid engine acceleration, the correspondingly rapid acceleration of the high pressure spool can result in a relatively low bleed air pressure between the twin compressor stages, with the undesirable result that sump seal oil leakage can occur.
There exists, therefore, a need for further improvements in bleed air buffer seal arrangements for use in gas turbine engines of the multiple spool type, to positively prevent sump seal oil leakage throughout the range of normal engine operating conditions. The present invention fulfills this need and provides further related advantages.